Heart failure afflicts more than 6 million Americans. Despite advances in heart failure therapy, there is no clinically available intervention to reverse underlying heart muscle injury. In recent years, stem cell therapy has been proposed as a way to regenerate the damaged tissue. It has become clear that stem cells' capacity to heal derives in large part from their ability to produce growth factors that accelerate the body's own repair mechanisms. However, current methods to administer stem cells to the heart, including intracoronary infusion or direct intramyocardial injection, are not conducive to the sustained production of beneficial growth factors. Rapid cell dilution, washout, and immune attack limit retention of viable stem cells, and, consequently, diminish the ability of the stem cells to produce sufficient growth factors to have desirable clinical effects.